Magnetic porous particles and method of making

ABSTRACT

The invention provides porous particles that produce a predetermined optical response and that may be manipulated magnetically. A preferred particle of the invention has a porous structure that produces a predetermined optical response and magnetic material adhered to the particle. Another preferred particle is amphiphilic. The optical response provided by a particle of the invention enables particles of the invention to be used in sensing, labeling, signaling, display and many other applications. The magnetic nature of the present magnetic particles permits the particles themselves to be manipulated, e.g., vibrated, moved and re-oriented. The porous particles can also be used to control, move, and/or deliver small volumes of liquids and solids associated with the particles.

PRIORITY CLAIM

Applicants claim priority benefits under 35 U.S.C. §119 on the basis ofPatent Application No. 60/589,171, filed Jul. 19, 2004.

STATEMENT OF GOVERNMENT INTEREST

The invention was made with Government support under NIH Grant NumberN01-CO-37117 awarded by the National Institutes of Health, and under AirForce Office of Scientific Research Grant Number F49620-02-1-0288. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

A field of the invention is optoelectronics. Another exemplary field ofthe invention is optical structure fabrication. Another exemplary fieldof the invention is encoding. Additional exemplary fields of theinvention include the life sciences, security, product marking, foodprocessing, agriculture, and chemical detection. Still another field ofthe invention is microfluidics. Yet another exemplary field of theinvention is transport of liquids and solids using magnetic fields.

BACKGROUND

Porous particles constructed from electrochemically etched porousmaterials, such as silicon, have widespread application inoptoelectronics, chemical and biological sensors, high-throughputscreening, and drug delivery applications. These porous particles areespecially advantageous because of the relative ease with which theoptical properties, pore size, and surface chemistry can be manipulated.Moreover, position, width, and intensity of spectral reflectivity peaksmay be controlled by the current density waveform and solutioncomposition used in the electrochemical etch, thus rendering possiblethe preparation of films of porous particles that display any colorwithin the visible light band with high color saturation, which is adesirable feature for information displays.

A well-appreciated need for labeling exists in society. Labeling is afundamental basis for tracking and identifying. Encoding can be used asa form of labeling understood by persons or equipment, as in the case ofbar coding. At the microscale, however, labeling/encoding itself becomesdifficult.

Porous particles and films constructed from electrochemically etchedporous materials have provided powerful methods for labeling andencoding. Porous particles and films and methods using such aredisclosed in 1) U.S. Published Patent Application 20050042764, entitled“Optically encoded particles” to Sailor et al., published Feb. 24, 2005;2) U.S. Published Patent Application 20050009374, entitled “Directpatterning of silicon by photoelectrochemical etching”, to Gao, et al.,published Jan. 13, 2005, now U.S. Pat. No. 7,433,811, issued Oct. 7,2008; 3) U.S. Published Patent Application 20030146109 entitled “Porousthin film time-varying reflectivity analysis of samples,” to Sailor, etal. published Aug. 7, 2003, now U.S. Pat. No. 7,042,570, issued May 9,2006; 4) PCT Application PCT/US04/043001, entitled “Optically encodedparticles, system and high throughput screening, to Sailor et al, filedDec. 21, 2004, now U.S. Publication No. US-2007-0148695-A1, publishedJun. 28, 2007; 5) PCT Application PCT/US04/042997, entitled “Opticallyencoded particles with grey scale spectra,” to Sailor et al, filed Dec.21, 2004, now U.S. Publication No. US-2007-0051815-A1, published Mar. 8,2007; and 6) PCT Application PCT/US04/26572, entitled, “Photonic SensorParticles and Fabrication Methods”, to Sailor, et al filed Aug. 13,2004, now U.S. Pat. No. 7,167,605, issued Jan. 23, 2007.

SUMMARY

The invention provides particles that produce a predetermined opticalresponse and that may be manipulated magnetically. A preferred particleof the invention has a porous structure that produces a predeterminedoptical response and magnetic material adhered to the particle. Anotherpreferred particle is amphiphilic. The optical response provided by aparticle of the invention enables particles of the invention to be usedin sensing, labeling, signaling, display and many other applications. Anexemplary porous surface is a photonic crystal surface. The magneticnature of the present magnetic particles permits the particlesthemselves to be manipulated, e.g., vibrated, moved and re-oriented. Theparticles can also be used to control, move, and/or deliver smallvolumes of liquids and solids associated with the particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a preferred method for forming magnetic porousparticles according to the invention;

FIG. 2 illustrates a preferred method for manipulating magnetic porousparticles according to the invention;

FIG. 3 shows white light reflection spectra taken from both sides of amagnetic porous silicon photonic crystal produced by experiments;

FIG. 4 is a plot of optical response versus time that illustrates aflipping rate of magnetic photonic crystal particles, measured bymonitoring reflected laser light (632 nm, He/Ne) from a 100 micron(approx) particle;

FIG. 5 illustrates a preferred method for forming magnetic porousparticles having sides with separate affinities according to theinvention;

FIG. 6 shows reflectivity spectra of amphiphilic magnetic porousphotonic crystal particles before and after delivery of payload,demonstrating fluidic transport by particles of the invention; and

FIGS. 7A-7C show reflectivity spectra of two sets of amphiphilicmagnetic porous photonic crystal particle chaperones involved in mixingchemicals to perform a reaction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides magnetic porous particles. Particles of theinvention have a porous surface and a magnetic surface. The surface hasa predetermined optical response characteristic of a porosity in thematerial, e.g., silicon or another semiconductor. An exemplary poroussurface is a photonic crystal surface. The optical response provided bya particle of the invention enables particles of the invention to beused in sensing, labeling, signaling, display and many otherapplications. The magnetic surface enhances the known uses for porousstructures, and also provides for additional uses that will berecognized by artisans with reference to the detailed descriptionherein. Generally, the magnetic nature of the present magnetic porousparticles permits the particles themselves to be manipulated, e.g.,vibrated, moved and re-oriented. The magnetic porous particles can alsobe used to control, move, and/or deliver small volumes of liquids andsolids associated with the particles.

Magnetic porous particles of the invention can be fabricated fromsubstrates of semiconductor or other suitable solid-state materials.Preferably, the substrate is a single-crystal material. A silicon (Si)crystal is a preferred substrate. Single crystal materials are preferredmaterials.

Embodiments of the invention provide magnetically switchable,micron-sized porous particles, and a method for fabrication of the same.More specifically, embodiments of the invention include porous particleshaving magnetic nanoparticles adhered thereto.

Particles of the invention are applicable to a variety of industries,including but not limited to drug discovery, biological screening,chemical screening, biological labeling, chemical labeling, in vivolabeling, security identification, signaling, displays, microfluidicsand product marking. Various attributes of the particles and methods ofthe invention enable a wide range of applications in various industries.The small size of the particles facilitates ready incorporation intovarious hosts, e.g., products. test kits, assays, powders (such asexplosives for identification), pastes, liquids, glass, paper, and anyother host or system that can accept small particles. In vivo detectionis enabled by biocompatible particles of the invention, such as siliconparticles, which may then be queried, for example, through tissues usingnear infrared and infrared wavelengths that penetrate tissues.

Additionally, the particles of the invention have application to opticalsignaling and display applications. Particles of the invention provide aswitchable optical display capability. The porous surface of a particleof the invention provides a strong predetermined optical responsecharacteristic of the porosity of the material, and the magnetic surfacedoes not. This is the basis for display or signaling. The opticalresponse of an individual particle of the invention may be tailored tohave a particular optical characteristic, e.g., to provide an opticalresponse of a certain wavelength. Thus, for example, under control of amagnetic field, a particle or group of particles can be caused toproduce an optical response of a particular color. Other particles,having a different photonic structure, may produce an optical responseof a different color. The optical response of a particle may change inthe presence of an analyte, permitting analyte detection.

Embodiments of the invention enable manipulation of liquids at themicroscale without necessitating the addition of ions or otherimpurities to the bulk liquid in order to induce transport. Liquids mayalso be manipulated outside of traditional microfludic structures (aswell as within such structures).

In a preferred embodiment magnetic porous particle of the invention,superparamagnetic nanoparticles, e.g., magnetite (Fe₃O₄), can beincorporated into the porous nanostructure. Preferred embodimentseparate affinity particles have a first surface having a firstaffinity, e.g., a hydrophilic surface, and a second surface having asecond affinity, e.g., a hydrophobic surface, allowing the materials tochaperone microliter-scale liquid droplets by application of an externalmagnetic field. The optical reflectivity spectrum response of thedisplays a peak that serves to identify the particle and theparticle-liquid interaction. Other exemplary affinities can be createdby, for example, incorporating a specific antibody, oligonucleotide orligand to provide affinity for a specific antigen, oligonucleotide, orreceptor.

In a preferred embodiment, photonic crystals particles are made fromporous Si are with amphiphilic properties. Magnetic nanoparticles ofFe₃O₄ can be incorporated into the porous nanostructure. When placed ina two-phase liquid such as dichloromethane/water, these materials willaccumulate and spontaneously align at the interface. If one of theliquids exists as small droplets in the other liquid, the particles canencapsulate the droplets, allowing the materials to chaperonemicroliter-scale liquid droplets by application of an external magneticfield. The optical reflectivity spectrum of the porous particle displaysa peak that serves to identify the particle and the particle-liquidinteraction.

In a preferred magnetic porous particle fabrication method of theinvention, a multilayered porous dielectric mirror (rugate filter) isfirst etched into a single-crystal Si substrate. The film is thenhydrosilylated to generate a chemically stable hydrophobic mirror. Asecond rugate filter with a different periodicity is etched into thesubstrate, immediately beneath the first rugate filter. The porouscrystal film is released from the substrate. The porous crystal film isthen divided into micron-sized particles by ultrasonication. Magneticparticles are infused into the second filter and impart a hydrophiliccharacter to the second layer.

A particle formed by creating a particular porosity in a material, e.g.,silicon, is a foundation for a method of fabrication of a magneticporous particle of the invention. The particular porosity will governthe nature of the optical response of a magnetic porous particle of theinvention. Particles may be encoded with different optical structures.Different optical structures can be etched prior to the adhering of themagnetic nanoparticles. Processes for creating different opticalstructures through creating porosity are described, for example, U.S.Published Patent Application 20050042764, entitled “Optically encodedparticles” to Sailor et al., published Feb. 24, 2005; 2) U.S. PublishedPatent Application 20050009374, entitled “Direct patterning of siliconby photoelectrochemical etching”, to Gao, et al., published Jan. 13,2005, now U.S. Pat. No. 7,433,811, issued Oct. 7, 2008; 3) U.S.Published Patent Application 20030146109 entitled “Porous thin filmtime-varying reflectivity analysis of samples,” to Sailor, et al.published Aug. 7, 2003, now U.S. Pat. No. 7,042,570 issued May 9, 2006;4) PCT Application PCT/US04/043001, entitled “Optically encodedparticles, system and high throughput screening, to Sailor et al, filedDec. 21, 2004, now U.S. Publication No. US-2007-0148695-A1, publishedJun. 28, 2007; 5) PCT Application PCT/US04/042997, entitled “Opticallyencoded particles with grey scale spectra,” to Sailor et al, filed Dec.21, 2004, now U.S. Publication No. US-2007-0051815-A1, published Mar. 8,2007; and 6) PCT Application PCT/US04/26572, entitled, “Photonic SensorParticles and Fabrication Methods”, to Sailor, et al filed Aug. 13,2004, now U.S. Pat. No. 7,167,605, issued Jan. 23, 2007.

The porosity of particles of the invention can have a complex structurethat enables coding, for example. With varying porosity and multipleporosities in different layers, complex codes can be encoded in theporosity of a particle of the invention. Example codes realizablethrough control of the porosity of a particle are disclosed in U.S.Published Patent Application 20050042764, entitled “Optically encodedparticles” to Sailor et al., published Feb. 24, 2005; and PCTApplication PCT/US04/042997, entitled “Optically encoded particles withgrey scale spectra,” to Sailor et al. filed Dec. 21, 2004 now U.S.Publication No. US-2007-0051815-A1, published Mar. 8, 2007.

A preferred method of formation and a porous magnetic particle of theinvention are illustrated in FIG. 1. A substrate 10, e.g., silicon oranother semiconductor, is etched by a sinusoidal anodic etch. The etchis controlled to produce a crystal film 12 having a porosity having acharacteristic optical response. The porous film 12 is released from thesubstrate 10. The release can be achieved, for example, by applicationof a current pulse. Magnetic nanoparticles 14, such as Fe nanoparticlescommercially available from Nanomat, Inc. of North Huntingdon, Pa., areadhered to one side of the porous film 12. Upon drying, the magneticnanoparticles 14 adhere sufficiently to the porous film 12. The film isthen divided, for example by fracturing, into small magnetic porousparticles 16. The particles 16 have, on one side, the porous filmsurface 12 and, on the other side, the magnetic material 14.

The magnetic porous particles 16 have microscopic dimensions. Preferredembodiment particles are formed from silicon, which is a biocompatiblematerial, have a magnetic coating of Fe. This results in a magneticcoating on one side, which exhibits an insubstantial optical response,and a porous film on the other side, which exhibits a strong opticalresponse, e.g., an intensely colored optical response. Other magneticmaterials may also be used, e.g., rare earth magnetic materials,gadolinium, or nickel.

FIG. 2 illustrates an example method for manipulation of magnetic porousparticles 16 of the invention. A magnetic porous particle 16 (orparticles) of the invention is placed in an oscillating magnetic fieldcreated by a magnetic field source 18. The oscillating magnetic field H₀causes the small particle 16 to vibrate, and at predetermined values ofmagnetic field strength and oscillation frequency, the porous particlescan be induced to reorient, i.e., flip over to juxtapose the position ofthe magnetic side and the porous side of the particle 16.

The reflection of a laser beam impinging on particles of the inventioncan be used to quantify the rate of switching, with larger switchingrates in smaller particles. Substantial switching rates are possible. Inan exemplary experiment, switching of the particles at a rate of 175 Hzwas demonstrated.

In addition to the vibration and reorientation of particles 16, magneticfield(s) may also be used to move particles 16 from one location toanother, such as in a fluid medium, e.g., a microfluidic channel or avolume of liquid. Movement of particles by application of a magneticfield can be useful, for example, in optoelectronics, chemical andbiological sensors, high-throughput screening, and drug deliveryapplications.

Magnetic porous particles may be used to bind and then releasematerials, including liquids and solids. Thus, other exampleapplications of particles of the invention include microfluidics;transport of liquids and solids using magnetic fields; controlledmanipulation of liquid or solid micro or nano particles; delivery,targeting, and controlled release of therapeutic and/or diagnosticreagents in a patient; high throughput screening of molecules forgenomics, proteomics, drug discovery applications; and controlledmanipulation of liquids containing cells, proteins, or other biologicalsystems.

The formation method of FIG. 1 has been demonstrated with experiments.The invention is not limited to the experiments, but additionalembodiments and inventive features will be apparent to artisans byreference to the experimental results that will now be described.

In the experiments consistent with the formation method of FIG. 1,multilayered porous silicon films were prepared by electrochemical etchof Si wafers (degenerate p-type, B doped, <1 mOhm-cm resistivity,polished on the (100) face, obtained from Siltronix, Inc.). The etchingsolution consisted of a 1:3 by volume mixture of absolute ethanol(Aldrich Chemicals) and aqueous 49% HF (Quantum Chemicals).Galvanostatic etching was carried out in a Teflon cell using atwo-electrode configuration. The current density was modulated with asine wave (typically between 12 and 35 mA/cm², 8.5 s periodicity, 70repeats) to generate a periodically varying porosity gradient. The filmswere removed from the substrate by applying a 180 second pulse ofcurrent density 460 mA/cm² while the film was still immersed in theelectrolyte solution. The film was then rinsed with ethanol and dried inair. Fe nanoparticles (8 nm average diameter, obtained from NanomatInc.), suspended in hexane were cast onto the porous Si multilayer filmand the hexane was allowed to evaporate in air. The porous Si/Fecomposite film was then fractured into small particles, yieldingmicroscopic magnetic photonic crystals.

Magnetic porous particles produced by the above process were placed on aglass plate over a wire coil electromagnet wound around an iron core.The electromagnet was powered by a EG&G Princeton Applied Research 363potentiostat operating in constant current mode, with an external sourcewaveform supplied by the sinusoidal output from a Tektronix CFG 250function generator. The waveform supplied ±400 mA to the magnet coil,producing a maximum field of approx. 600 mT. To measure the flippingrate of the magnetic particles in the oscillating magnetic field, theparticles were illuminated with a 10 mW He/Ne laser and the reflectedlight intensity was measured as the photocurrent from an EalingElectro-Optics Si photodiode. The photodiode signal was amplified with aStanford Research Systems SR560 preamplifier, and recorded on aTektronix TDS 520 digitizing oscilloscope.

Optical reflectivity spectra were obtained using a tungsten lightsource, optical microscope, and an Ocean Optics S2000 silicon CCDspectrometer fitted with a fiber optic input. The light collection endof the fiber optic was positioned at the focal plane of an opticalmicroscope, allowing acquisition of spectra from a nominal spot size of1 mm.

Photographs of ferromagnetic porous Si photonic crystal after formationrevealed a green side that reflected the color characteristic of thephotonic crystal, while the bottom side, coated with Fe nanoparticles,appeared black. The particles were flipped over in the experiments byapplication of an oscillating magnetic field.

Scanning electron microscope (SEM) images were obtained using an FEIQuanta 600 instrument operating at an accelerating voltage of 20 kV. AnOxford Instruments Energy Dispersive X-ray Spectrometer (EDS) was usedto obtain qualitative elemental analysis of the Si and Fe nanoparticlelayers. Cross-sectional scanning electron micrograph images(backscattered electron image) of a ferromagnetic porous Si photoniccrystal produced by the above process before removal from the Sisubstrate showed a layer of Fe nanoparticles on top of a porous Simultilayer.

FIG. 3 shows white light reflection spectra taken from both sides of amagnetic porous Si photonic crystal produced by experiments. The intenseand sharp reflectance peak expected for a rugate filter is observed onthe photonic crystal side (solid trace, top), whereas a much weakerreflectance peak is detected on the side containing an over coating ofFe nanoparticles (dashed trace, bottom).

FIG. 4 is a plot of optical response versus time that illustrates aflipping rate of a magnetic photonic crystal particles, measured bymonitoring reflected laser light (632 nm, He/Ne) from a 100 micron(approx) particle. The magnetic photonic crystal (λ_(max) of thephotonic feature 632 nm) was subjected to an alternating magnetic fieldgenerated by an electromagnet. The reflected light was detected with anamplified photodiode connected to a digital oscilloscope. The bottomtrace is recorded at a switching rate of 82 Hz. The top trace is thesame particle recorded at a switching rate of 175 Hz. The bottom traceis offset by −0.2 V along the y-axis.

Another specific embodiment magnetic porous particle of the inventioncan self-assemble at a hydrophobic/hydrophilic interface. Such particlescan effect controlled manipulation of small volumes of liquids, whichhas application in microfluidics for many high throughput analyses andmicro assays.

FIG. 5 shows a preferred embodiment method of formation of magneticporous particles with separate affinities. The process begins as theprocess does in FIG. 1, and like reference numerals will be used toindicated like elements in FIG. 5. A substrate 10, e.g., silicon oranother semiconductor, is etched by a sinusoidal anodic etch. The etchis controlled to produce a crystal film 12 having a porosity having acharacteristic optical response. In a preferred embodiment, the etch iscarried out to achieve a sinusoidal variation in refractive index thatresults in a Rugate filter, which produces a sharp resonance feature inthe optical reflectivity spectrum of the film, whose wavelength isdetermined by the periodicity and current limits used in the etch. Thefilm 12 is then made to have a first affinity, e.g., by thermallyhydrosilylating with 1-dodecene, generating a modified film 12 a that isa chemically stable hydrophobic mirror. A second etch is carried out toform a second porous crystal film 18 having a different periodicity fromthe first film 12 a. In a preferred embodiment, the etch is conductedsuch that the film 18 forms a second rugate filter, immediately beneaththe first. Releasing and dividing the films 12 a, 18 from the substrate10 forms particles 20, having a porous crystal side that has a firstaffinity, e.g. is hydrophobic (in accordance with properties of the film12 a), and a side that has a second affinity, e.g. is hydrophilic (inaccordance with the properties of the film 18). Magnetic nanoparticlesare then infused into the one of the opposite flat sides 20 a and 20 bof the particle 20, e.g., side 20 a of the particles to create magneticporous particles 22 that have the separate affinities. Preferredembodiment separate affinity particles have a first surface having afirst affinity, e.g., a hydrophilic surface, and a second surface havinga second affinity, e.g., a hydrophobic surface, allowing the materialsto chaperone microliter-scale liquid droplets by application of anexternal magnetic field. The optical reflectivity spectrum response ofthe displays a peak that serves to identify the particle and theparticle-liquid interaction. Other exemplary affinities can be createdby, for example, incorporating a specific antibody, oligonucleotide orligand to provide affinity for a specific antigen, oligonucleotide, orreceptor.

Experiments were conducted to demonstrate the method of FIG. 5. Theinvention is not limited to the experiments, but additional embodimentsand inventive features will be apparent to artisans by reference to theexperimental results that will now be described.

Multilayered porous Si dielectric mirrors were prepared byelectrochemical etch of a single-crystal, (100)-oriented p-type Si waferwith a resistivity between 0.08 to 0.2 mOhm-cm, by application of asinusoidal current waveform oscillating between 13.3 and 39.8 mA/cm².The periodicity of the waveform was 8 sec, repeated for 100 cycles in a3:1 (v:v) aqueous 49% HF:ethanol solution. Thermal hydrosilylation wasaccomplished by placing the porous Si sample in liquid 1-dodecene (95%,Sigma-Aldrich Chemicals) in a Schlenk flask, degassing the liquid with 3successive freeze-pump-thaw cycles, and then heating at 120° C. undernitrogen for two hours. A second rugate filter with a periodicity of 10seconds was then etched into the substrate, immediately beneath thefirst layer for the same number of repeats. The porous Si structure wasthen removed from the crystalline Si substrate by application of acurrent pulse of ca. 15 mA/cm² in a 1:13.5 (v:v) aqueous 49% HF:ethanolsolution for ca. 1 min, and the freestanding film was rinsed withethanol and dried in an oven at 100° C.

Magnetite (Fe₃O₄)for infusion was prepared. In a representativepreparation, 22 100 mL of a 1M aqueous NH₄OH solution is added over aperiod of ten minutes to 10 mL of a 4:1 solution of 1M FeCl₃ and 2MFeCl₂ in water. The resulting black precipitate (magnetite) was filteredand then re-dispersed in 10 mL of 40 wt. % aqueous tetrabutylammoniumhydroxide. Dynamic light scattering measurements indicated that theparticles were ca. 30 nm in diameter.

The magnetite particles were then adhered to (trapped in) the film. Thefreestanding film was fractured by ultrasonication in ethanol and theresulting micro particles allowed to settle. The supernatant wasremoved, and the aqueous magnetite colloidal suspension was added. Thehigh pH of the magnetite suspension spontaneously induces oxidation ofthe second porous Si layer, presumably trapping the magnetitenanoparticles in an oxide matrix. After isolation from the solution withthe aid of an applied magnetic field, the microparticle composites arethermally oxidized to more completely trap the magnetite. Oxidation ofthe porous Si films was confirmed by Fourier transform infrared (FTIR)spectroscopy, and the presence and disposition of magnetite in theporous Si film is determined by FTIR and elemental mapping of iron in across-sectional electron microscope image using energy dispersive x-rayspectroscopy. Thermal oxidation preferentially produces oxide on themore reactive, hydrogen-terminated porous Si layer. The oxide impartshydrophilic character to the second mirror while the hydrosilylated sideretains its hydrophobic nature, allowing the particles to align at anaqueous/organic liquid interface.

To collect and oxidize particles, after stirring for 5 min, the magneticporous Si photonic crystal particles were isolated from solution byremoval with a bar magnet. While still being held in the flask with amagnet, the particles were rinsed several times with acetone to removefree magnetite nanoparticles. The isolated magnetic microparticles werethen thermally oxidized in air at 100° C. for 1-2 h.

In a basic demonstration of a microfluidic application, the bifunctionalmagnetic microparticles were used to chaperone droplets of waterimmersed in an organic phase between a source and a drain. Themicroparticles spontaneously orient at the interface of a 30 microliterwater droplet, with the hydrophilic side facing the aqueous phase. Theinterfacial adhesive forces are strong enough to allow the particles tomove the droplet when a magnetic field is applied. When the dropletassembly is brought into contact with a glass capillary, water isremoved from the assembly by capillary action. The magnetic porous Sichaperones can then be moved to a stainless steel syringe needle andrefilled with liquid, completing the transport cycle.

The reflectivity spectrum from the bifunctional mirrors provides asignal that that can be used to identify the payload. The position ofthe spectral peaks in porous Si dielectric mirrors is a strong functionof the average refractive index of the layers, and shifts in thewavelength of the spectral features have been shown to provide a verysensitive transduction modality for sensing condensable liquids,proteins, DNA, and other molecules. In example magnetic amphiphilicporous photonic crystal particles of the invention, the hydrosilylatedlayer displays a spectral wavelength maximum of 540 nm that shifts to579 nm when it is immersed in hexane. The opposite side of the particledisplays a band at 660 nm in air that shifts to 760 nm when immersed.The porous Si photonic crystals contain enough repeat layers (100) thatcross-talk between opposite sides is not readily observed in thespectra.

FIG. 6 shows reflectivity spectra of magnetic porous photonicamphiphilic particles before and after delivery of payload. In the Atrace of FIG. 6, the reflectance spectrum was obtained from particlesassembled on a water drop immersed in hexane. In the B trace of FIG. 6,the reflectance spectrum was obtained from the assembly after deliveryof the aqueous payload to a capillary. Spectra were acquired using anOcean Optics SD2000 CCD (charge-coupled device) spectrometer fitted withfocusing optics and using a tungsten light source coaxial with thedetection optics. The detection optics were positioned as indicated inthe inset.

The approximate spherical shape of the water droplet/chaperone assemblyleads to a strong reflection from the hydrophobic side over a largerange of incident angles FIG. 6, trace a. Upon delivery of the aqueouspayload, the sphere is deflated and reflection from the assembly isdramatically reduced in intensity, FIG. 6, trace b. These spectralcharacteristics can thus indicate the identity of the chaperones and thestatus of the chaperoned droplet.

FIGS. 7A-7C show reflectivity spectra of two sets of magneticamphiphilic porous Si chaperones involved in mixing chemicals to performa reaction. The inset of each spectrum shows a photograph of theexperiment representing that stage of the reaction. The FIG. 7Areflectance spectrum was obtained from particles assembled on a waterdroplet (4 mm diameter) containing Ag+ (aq). The particles on thisdroplet contained a spectral code consisting of two peaks. In FIG. 7B,the reflectance spectrum was obtained from a water droplet containingI(aq) and the particles had a separate spectral code consisting of asingle peak. In FIG. 7C, the reflectance was spectrum obtained fromwater droplet after the two drops were combined under the force of amagnet, showing the superposition of the two spectral codes. The AgI(s)product of the reaction is apparent in the image as a white cloud insidethe drop. Water droplets were immersed in octadecene throughout theexperiments.

The method presented here provides a general technique for manipulatingsmall volumes of liquids without a microfluidic container. Because themagnetic porous Si particles adhere to the surface of the water drop,they do not require a specific payload composition such as a high ionicstrength in order to effect liquid motion. The electrochemical synthesisof porous Si photonic crystals allows the incorporation of spectral barcodes (see, e.g., U.S. Published Patent Application 20050042764,entitled “Optically encoded particles” to Sailor et al., published Feb.24, 2005), allowing the possibility of distinguishing between multipledistinct liquid drops in combinatorial assays. Since either water or anorganic liquid can be encapsulated by the amphiphilic particles, a widerange of inorganic or organic molecules can be manipulated for thepurpose of carrying out micro reactions. Mammalian or bacterial cellscan also be incorporated in this system for cell-based assays. Inaddition, a wide range of molecules can be incorporated within theporous nanostructure of the magnetic porous Si particles, allowingcontrolled manipulation of very small amounts of chemical reagents.

While specific embodiments of the present invention have been shown anddescribed, it should be understood that other modifications,substitutions and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

Various features of the invention are set forth in the appended claims.

1. A method of fabricating magnetic porous particles, the methodcomprising steps of: providing a substrate; etching the substrate tocreate a film having a porous photonic structure; releasing the filmfrom the substrate; dividing the film into particles; and magnetizingonly one side of the film prior to said step of dividing or only oneside of the particles after said step of dividing.
 2. The method ofclaim 1, wherein said step of magnetizing comprises adhering magneticnanoparticles to the only one side of the film prior to said step ofdividing.
 3. The method of claim 2, wherein said step of adheringcomprises depositing magnetic nanoparticles on or within the pores ofthe only one side of the film.
 4. The method of claim 1, furthercomprising applying a magnetic field to the particles to manipulate theparticles.
 5. The method of claim 1, wherein the substrate comprises asilicon substrate.
 6. The method of claim 3, wherein the magneticnanoparticles comprise Fe nanoparticles.
 7. The method of claim 1,wherein said step of magnetizing comprises infusing magnetic materialinto the pores of the only one side of the particles after said step ofdividing.
 8. The method of claim 7, further comprising steps of: aftersaid step of etching, treating the film to create a first affinity on aside opposite said only one side of said film; prior to said step ofreleasing, further etching the substrate to create a different porousphotonic structure and a second affinity on said only one side of saidfilm.
 9. The method of claim 8, wherein said step of infusing comprisesinfusing magnetite into the only one side of the particles after saidstep of dividing.
 10. A magnetic porous film particle, the particlecomprising: a porous film with opposite flat surfaces, a first one ofsaid opposite flat surfaces having a porosity that produces apredetermined optical response; and a second one of said opposite flatsurfaces being a magnetic surface that has an insubstantial or nooptical response.
 11. The particle of claim 10, wherein one of saidmagnetic surface and said porous surface is hydrophobic and the other ishydrophilic.
 12. The particle of claim 10, wherein said magnetic surfacecomprises a second porous surface having a second porosity, said secondsurface having adhered or infused magnetic material.
 13. A magneticporous particle, the particle comprising: porous material having aporosity that produces a predetermined optical response; and magneticmaterial adhered to or infused in only one side of the porous material,wherein said magnetic surface comprises a second porous surface having asecond porosity, said second surface having adhered or infused magneticmaterial and the magnetic material comprises magnetic nanoparticles. 14.The particle of claim 13, wherein the magnetic nanoparticles compriseFe₃O₄ nanoparticles.
 15. A magnetic porous particle, the particlecomprising: porous material having a porosity that produces apredetermined optical response; and magnetic material adhered to orinfused in only one side of the porous material, wherein the magneticmaterial comprises magnetic nanoparticles.
 16. The particle of claim 15,wherein the magnetic nanoparticles comprise Fe₃O₄ nanoparticles.
 17. Amethod of producing a display, the method comprising magneticallymanipulating a particle of claim 15 to selectively produce one of thepredetermined optical response and the insubstantial or no opticalresponse.
 18. The method of claim 17, further comprising a step ofswitching, by magnetically manipulating, between the predeterminedoptical response and the insubstantial or no optical response.
 19. Amethod of fabricating magnetic porous particles, the method comprisingsteps of: electrochemically etching a first side of semiconductor waferby application of a sinusoidal current waveform to the wafer when it isin an etching solution to create a first porous surface; hydrosilylatingthe first porous surface to make the first porous surface hydrophobicand chemically stable; electrochemically etching the semiconductor waferimmediately beneath the first porous surface to create a second poroussurface that is hydrophilic; releasing a porous film having the firstand second porous surfaces from the semiconductor wafer; magnetizing thesecond porous surface by placing the porous film in a magnetitenanoparticle suspension to induce oxidation of the second porous surfaceand adherence of magnetite nanoparticles to the second porous surface.20. The method of claim 19, further comprising a step of fracturing theporous film into particles.
 21. The method of claim 20, wherein saidstep of fracturing comprises ultrasonication.
 22. The method of claim21, further comprising a step of thermally oxidizing the second poroussurface to more completely trap the magnetite nanoparticles.